Prevent a race condition between the script engine backup thread and script removal by locking on the script's EventQueue and only proceeding if it's flagged as still running.

Relates to http://opensimulator.org/mantis/view.php?id=7407

OpenSim/Region/ScriptEngine/Interfaces/IScriptInstance.cs

@@ -178,8 +178,9 @@ namespace OpenSim.Region.ScriptEngine.Interfaces
         /// <param name="timeout"></param>
         /// How many milliseconds we will wait for an existing script event to finish before
         /// forcibly aborting that event.
+        /// <param name="clearEventQueue">If true then the event queue is also cleared</param>
         /// <returns>true if the script was successfully stopped, false otherwise</returns>
-        bool Stop(int timeout);
+        bool Stop(int timeout, bool clearEventQueue = false);
 
         void SetState(string state);